How much time will it take for an amount of $\text{Rs. }450$ to yield $\text{Rs. }81$ as interest at $4.5\%$ per annum of simple interest?

Given parameters:
* Principal ($P$) $=\text{Rs. }450$
* Simple Interest ($\text{SI}$) $=\text{Rs. }81$
* Rate of Interest ($R$) $=4.5\%$ per annum

The formula for simple interest is:

$$\text{SI}=\frac{P\times R\times T}{100}$$

Substituting the given values to solve for time ($T$):

$$81=\frac{450\times 4.5\times T}{100}$$

$$81=4.5\times 4.5\times T$$

$$81=20.25\times T$$

$$T=\frac{81}{20.25}=4\text{years}$$